Ohmic contacts to silicon

ABSTRACT

Ohmic contact to relatively high resistivity n-type silicon is made by forming a thin layer of platinum on the silicon surface by means of chemical vapor deposition (CVD) using a phosphorus compound of platinum. The structure then is sintered by heating at about 700* C. for from five to ten minutes thereupon forming a good ohmic contact of the platinum silicide type without the necessity of the separate impurity diffusion treatment usually required to increase the surface impurity concentration.

United States Patent 1191 Rand [ June 17, 1975 [73] Assignee: Bell Telephone Laboratories,

Incorporated, Murray Hill, NJ.

[22] Filed: Dec. 10, I973 [2]] Appl. No.: 422,996

3,590,471 7/1971 Lepselter 29/578 Primary Examiner-W. Tupman Attorney, Agent, or Firm-H. W. Lockhart [57] ABSTRACT Ohmic contact to relatively high resistivity n-type silicon is made by forming a thin layer of platinum on the silicon surface by means of chemical vapor deposition 52 us. Cl. 29/578; 29/590 VD) ing a p ph rus com ound of platinum. [51 1 Int. Cl B01 j 17/00 The structure th n is sinter d by heating at about 700 58 Field of Search 29/578, 589, 590 for from fi o en minutes thereu on forming a good ohmic contact of the platinum silicide type with- [5 Refe e Ci d out the necessity of the separate impurity diffusion UNITED STATES PATENTS treatment usually required to increase the surface im- 3,274,e 70 9/!966 Lepselter 29/578 Pumy Conccmmmm' 3,545,076 12/1970 Schulten 29/578 5 Claims, 2 Drawing Figures I PREPARE SILICON SEMICONDUCTOR WAFER FORM CONDUCTIVITY TYPE REGIO 11 IN WAFER FOR NS DEFINING ARRAY OF DEVICES ]I[ FORM MASK. DEFINE CONTACT AREAS m DEPOSIT PLATINUM BY CHEMICAL VAPOR PROCESS AND SINTER PATENTEIJJLIN 17 m5 13,8 89,3 59

I PREPARE SILICON SEMICONDUCTOR WAFER FORM CONDUCTIVITY TYPE REGIONS 11 IN WAFER FOR DEFINING ARRAY OF DEVICES m FORM DIFFUSION MASK,

DEFINE CONTACT AREAS FIG. PRIOR ART 11 CONTACT AREA DIFFUSION I ETCH CONTACT AREAS AND REMASK DEPOSIT PLATINUM OR PALLADIUM "SEE FILM BY CATHODIC SPUTTERING OR EVAPORATION AND SINTER REMOVE UNREACTED PLATINUM III AND APPLY METAL CONDUCTOR PATTERN I PREPARE SILICON SEMICONDUCTOR WAFER FORM CONDUCTIVITY TYPE REGIONS 11 IN WAFER FOR DEFINING ARRAY OF DEVICES FIG. 2

III FORM MASK. DEFINE CONTACT AREAS m DEPOSIT PLATINUM BY CHEMICAL VAPOR PROCESS AND SINTER REMOVE UNREACTED PLATINUM '1' AND APPLY METAL CONDUCTOR PATTERN I 1 QHMIC CONTACTS TO SILICON BACKGROUND OF THE; INVENTION .semblythen is heated at av temperature in excess of about 500C. for a short period which effects a sinter- .high concentration of significant impurity in the surface portion of the silicon to insure ohmic contact to the semiconductor body. For this purpose it is the general practice to carry out a separate impurity diffusion I to enhance the conductivity of such surface portion.

This step also requires an additional mask definition and etching treatment and, accordingly, adds to the complexity of the process. More importantly, the diffusion heat treatment requires temperatures in excess of 900 C. which may deleteriously affect otherportions of the device structure. It is therefore an object of this invention to simplify the fabrication of ohmic contacts, particularly to silicon semiconductor material of higher resistivities in the range from about 0.1 to l ohmcentimeters and above.

SUMMARY OF THE INVENTION.

In accordance with this invention platinum silicide ohmic contact ismade to n-type conductivity silicon semiconductor material, for example, in the specific range of 3 to 5 ohm-centimeters, such as is used typically for the fabrication of insulated gate field effect transistors. After the fabrication of the device structure by the formation of conductivity type zones by diffusion or ion implantation or'equivalent means, a mask is formed on the surface to which contact is to be made. Typically such a mask is delineated in a film of silicon dioxidein accordance with methods well known in the art. A film of platinum having a thickness of from about 500 to 700 angstroms then is deposited over the entire surface. Uniquely, however, this deposition is accomplished by means ofthe so-called chemical vapor deposition process. This process is based on a thermal dissociation of a platinum-phosphorus trifluoride compound which produces a film of platinum. Following the deposition of a platinum film in accordance with this technique, it is necessary only to carry out the sintering heat treatment at a temperature of about 700 C. for from 5 to minutes to produce a platinum silicide contact of the usual form which has excellent ohmic characteristics to the underlying silicon. This result is achieved without the usual separate high-temperature diffusion heat treatment of the'con'tact area using an n-type impurity to enhance theconductivity of the surface portions. There is thus eliminated not only the separate heat treatment but a mask definition step and the etching of the surface oxide formed during such heat treatment. In the absence of such conductivity enhancement, the platinum silicide formed by deposition methods other than the chemical vapor deposition will produce a Schottky-barrier, rectifying contact.

DESCRIPTION OF THE DRAWING The invention and its other objects and features will be more clearly understood from the following detailed description taken in conjunction with the drawing in which:

DETAILED DESCRIPTION In accordance with the prior art, a silicon semiconductor-wafer is processed for the fabrication therein of an array of insulated gate field effect transistors as described in FIG. 1. This process conforms, in general, to that described in the article by R. Burock, .I. R. DeBolt, and R. N. Parente entitled Manufacturing Beam Lead, Insulated-Gate, Field Effect Transistor (IGFET) Integrated Circuits, The Western Electric Engineer, Vol. XVII, No. 3 (July 1973) p. 3. Typically, as indicated in Block I, the IGFET array is formed from a monocrystalline silicon substrate material having a thickness of several mils and a diameter of 2 or 3 inches. Also, typically such a substrate may be of fairly low resistivity, for example 0.01 ohm-centimeter, containing antimony as a significant impurity to produce n+ conductivity. There is formed on one surface of this substrate an epitaxial film of silicon having a thickness of from seven to ten microns of arsenic doped n-type silicon with a resistivity for example of from 8 to 20 ohmcentimeters. Next, referring to Block II, in accordance with techniques well known in the art, there is formed in the epitaxial layer an array of insulated gate field effect transistors each comprising a source, a drain, and an intervening channel region. Then, as set forth in Block III, prior art practice calls for the formation of a silicon dioxide film mask which defines the contact zones on the epitaxial face of the slice. As described in Block IV, the wafer then is subjected to an n-type impurity diffusion heat treatment using, for example, phosphorus, to enhance the n-type conductivity of the exposed silicon surface. This diffusion heat treatment at a temperature of 900 C. or higher, most typically about 1000 C., may have a considerable effect upon the distribution of significant impurities already present in the semiconductor material for defining device structures, and also on the properties of the siliconsilicon dioxide interface in the gate region, which are known to affect the electrical behavior of the device. At the least, it is necessary in designing the device processing to take into account the effect of this final diffusion heat treatment, a consideration which, in some instances, may require a compromise of device characteristics.

Following the diffusion heat treatment, the oxide formed within the contact areas is removed typically by chemical etching and the surface is remasked as speci= fiedin Block V. The wafer is then subjected to plati= hu'rn deposition by a cathodic sputtering process, as de= ing radiation effects.

c'ant impurity'distribution within the semiconductor, in

- contrast to the possible effect of a lOO C. treatment electron beam'evaporation. These energetic deposition techniques, although generally satisfactory for bipolar devices, introduce a level of radiation damage in unipolar devices, such as lGFETs, which has led to the substi tution of palladium for platinum in such devices. Palla dium can be depositedby thermal evaporation, avoid- Next, the wafer is heated at about 550to 600 C. for

from five to ten minutes to effect an interdiffusion between the platinumand the silicon in the contact areas.

Then, as specified B lock VII, the unreacted platinum overlying the oxide film surface i's'r'emoved by etching in aqua regia. The fabrication process continues with formation of the metal conductonpattern in accordance with techniques known in the art.

In contrast with the foregoing prior art process, the technique in 'accordance'with this invention as disclosed in FIG. 2, utilizes similar starting material and initial processing steps to form the array of devices as described in Blocks 1 and Il. Howeverfonce the contact areas have bcen d'efined by a silicon dioxide rnask; as described in Block III, the method omits the "separate high temperature diffusion and proceeds directly-to deposition of'a platinum-film about 700 angstr'oms thick, formed as specif ed in Block W, by the chemical vapor deposition technique.

This process, in one specific form, employs the compound tetraki s (trifluoi'ophosphine) platinum, Pt( PF a colorless, volatile, andrea'sorlably stable liquid. The vapor "of this compound is picked up by gas 225 C. The carrier may be either hydrogen or an inert gas such a s'nitrogen'or'argon. In one specific embodiment using hydrogen and-a process temperature of 250 C. the deposition rate of platinum was between 50 and 100 A/minute.

Next, as further specified in' Block lV, the platinum silicide is formed by an interdiffusion' heat treatment at about 700 C. to produce an ohmic'contac't to the silicon. As in the prior art technique, the unreactedplatihum may be removed by aqua regia and the metal conductor pattern applied as shown in BlockV.

It is believed that the advantageous result enabling the elimination'of the separate diffusion heat treatment is a consequenceofjsomephosp'horus diffusion provided by the chemical vapor deposition technique in accordance with thisinve'ntion. Such diffusion generally is unexpected because of the small amount of residual phosphorus and because the vapor pressure of the phosphorus would tend to dissipate the element. It has, however/been found'to be a most advantageoustechnique for making ohmic contact to silicon of the higher resistivity ranges currently favored for'silicon semiconductor devices of the IGFET or MOSFET type. In particular, although a temperature of 700 C. has been indicatedfor thestep offormi'ng platinum silicide, temperatures inthe'range from 675 to 750 C; may be used satisfactorilylt isimportant to note that these for the phosphorus contact diffusion.

7 Moreoventhe deposition of platinum by the chemica'l va'por techniquein lieu of energetic processes including sputtering and electron gun evaporation avoids the consequent radiation damage produced by these processes. At the'pre'sent time, insofar as applicant is aware, these are the only useful alternatives to the chemical vapor process for depositing platinum of the required thickness.'The effects of radiation are particular ly deleterious to the thin insulating film of the gate area of semiconductor devices of the insulated gate or thin filmt ype. Thus, the process in=accordanceswith this invention enables the use of platinum for unipolar devices "similar to the bipolar device metallization, rather than requiring a departure to palladium."

Although the invention has been described in terms of certain specific embodiments, it will be understood to applytoea wide range of silicon semiconductor device fabrication. lts efficacy stems from the feature that excellent lowresistance contact can be made even to relatively high resistivity material simply by depositing the initial contact. metal by the chemical vapor process followed by the sintering heat treatment step. Thus the devices being fabricated may be other than lGFETs and contact may be to material formed in a variety of ways. v Moreover, within the ambit of this disclosure, chemicalvapor deposition is intended to encompass only the reactive process described hereinabove with respect to trifluorophosphine or trifluorophosphine hydride compounds. lt, will also be understood that relatively high resistivity material refers to semiconductor material which in the absence of the practice of vthis invention requires an impurity enhancement treatment to enable satisfactory ohmic contact.

ltshould be noted also that the vapor deposition process in accordance with this invention may be practiced with other metals than platinum that have trifluorophosphine compounds suchfas nickel, chromium tungsten, and molybdenum. I i

What is claimed is:

1. [nthe fabrication of a silicon semiconductor de- .vice which includes the step of introducing into a pori vapor deposition a thin layer of platinum, said process comprisingexposing said inaskedsurface portion toa gaseous ambient containing a vapor of a phosphorous temperatures have little or no effect upon the signifi-r the. platinumlayer has compound of platinum while said semiconductor body is at a temperature of between and 500 C., followed by-heating said body at a temperature of from about 675 C: to 750 C. for a period of from 5 to l0 minutes. .-i

2. The process in accordance with claim 1 in which a thickness of from about 500 to 700angstroms.

' 3. The process in acc ordancewith claim 2 in which i saidportion has a resistivityin the range from about 2 to ohms centimeters.

the temperature of the semiconductor body during the chemical vapor deposition step is about 225 C. and the deposition period is for about 5 to 10 minutes. 

1. IN THE FABRICATION OF A SILICON SEMICONDUCTOR DEVICE WHICH INCLUDES THE STEP OF INTRODUCING INTO A PORTION OF A PTYPE CONDUCTIVITY SILICON SEMICONDUCTOR BODY AN N-TYPE IMPURITY TO CONVERT THE CONDUCTIVITY TYPE OF SAID PORTION TO AN NTYPE CONDUCTIVITY ZONE, THE FURTHER STEPS OF FORMING A SILICON OXIDE MASK ON THE SURFACE OF SAID PORTION TO DEFINE A CONTACT AREA THERETO AND PROCEDDING DIRECTLY AS A NEXT STEP TO DEPOSIT ON SAID MASKED SURFACE PORTION BY THE PROCESS OF CHEMICAL VAPOR DEPOSITION A THIN LAYER OF PLATINUM, SAID PROCESS COMPRISING EXPOSING SAID MASKED SURFACE PORTION TO A GASEOUS AMBIENT CONTANING A VAPOR OF A PHOSPHOROUS COMPOUND OF PLATINUM WHILE SAID SEMICONDUCTOR BODY IS AT A TEMPERATURE OF BETWEEN 100* AND 500*C., FOLLOWED BY HEATING SAID BODY AT A TEMPERATURE OF FROM ABOUT 675*C. TO 750*C, FOR A PERIOD OF FROM 5 TO 10 MINUTES.
 2. The process in accordance with claim 1 in which the platinum layer has a thickness of from about 500 to 700 angstroms.
 3. The process in accordance with claim 2 in which said portion has a resistivity in the range from about 2 to 10 ohms centimeters.
 4. The process in accordance with claim 1 in which the phosphorous compound of platinum is tetrakis (trifluorophosphine) platinum, Pt (PF3)4.
 5. The process in accordance with claim 4 in which the temperature of the semiconductor body during the chemical vapor deposition step is about 225* C. and the deposition period is for about 5 to 10 minutes. 